The Andes didn’t form in a single event. The earliest mountain-building activity along South America’s western margin dates back roughly 500 million years, but the range as we recognize it today began taking shape around 70 to 80 million years ago and gained most of its dramatic height in bursts over the last 30 million years. That makes the Andes both ancient in their roots and surprisingly young in their current towering form.
Ancient Roots Along the Edge of Gondwana
Long before the modern Andes existed, the western edge of South America was already geologically active. Around 500 to 490 million years ago, during the Cambrian and Ordovician periods, a mountain-building event called the Famatinian orogeny created a belt of metamorphic and volcanic rock stretching more than 400 kilometers along what is now Argentina. This was driven by oceanic crust diving beneath the supercontinent Gondwana, the same basic process that builds the Andes today.
These “proto-Andean” events left behind basement rocks that still sit beneath parts of the modern range. Studies of the Andean belt suggest that roughly 40% of its total growth since the late Precambrian (around 750 million years ago) came from smaller landmasses colliding with and accreting onto South America’s margin. So the Andes weren’t built from scratch. They grew on top of a foundation that had been accumulating material for hundreds of millions of years.
The Modern Range Begins: Late Cretaceous Onward
The mountain-building cycle responsible for today’s Andes kicked off in the Late Cretaceous, roughly 70 to 80 million years ago. In the Northern Andes, from Peru to Colombia, uplift between about 72 and 60 million years ago was driven by the South American plate colliding with the Caribbean Arch. This “Peruvian” phase of deformation is the earliest recognized stage of the current Andean orogeny.
After that initial pulse, the range continued building through a series of distinct phases, each named for its timing and character. The Incaic phase occurred during the Eocene (roughly 55 to 34 million years ago), followed by at least five Quechuan phases spanning from the early Oligocene through the late Miocene. Each phase folded and deformed different layers of rock, progressively stacking the crust higher. The D1 phase in the early Oligocene folded sedimentary deposits in the Western Cordillera. The D3 phase in the middle Miocene deformed the high plateau known as the Altiplano. The later phases continued reshaping the landscape into the late Miocene, around 9 to 10 million years ago.
Growth Spurts, Not Gradual Rise
For a long time, geologists assumed the Andes rose gradually and steadily as the Nazca oceanic plate pushed beneath the South American plate. The expectation was that the mountains grew in lockstep with subduction, inching upward over tens of millions of years. The actual story turned out to be far more dramatic.
Research from the University of Rochester and the Florida Museum found that the Andes gained much of their height in rapid pulses separated by long stretches of relative stability. Significant uplift began between 30 and 20 million years ago. The range then plateaued before a burst of rapid rise occurred between 10 and 6 million years ago, when parts of the landscape shot up by 1.5 to 3.5 kilometers. In the southern Altiplano, the surface rose about 2.5 kilometers between 16 and 9 million years ago. In geological terms, gaining a kilometer of elevation over several million years counts as fast.
The pattern, as geologist Carmala Garzione of the University of Rochester described it, is one of “pulses of rapid surface uplift over several million years, separated by long periods of stable elevations.” One explanation for these bursts is that dense material accumulated deep beneath the crust over time, then suddenly broke away and sank into the mantle, causing the lighter crust above to bounce upward. Massive volcanic eruptions in the Altiplano-Puna region since about 11 million years ago may also have contributed, with hot magma from below pushing the surface higher.
Different Sections Rose at Different Times
The Andes stretch over 7,000 kilometers, and the range didn’t all rise at once. The Northern Andes saw their earliest uplift in the Late Cretaceous, around 70 million years ago. The Colombian Andes experienced rapid uplift of their Eastern Cordillera much later, between 5 and 2 million years ago, at rates of 0.6 to 3 millimeters per year. The Central Andes, including the Altiplano of Bolivia and southern Peru, underwent their most dramatic uplift between about 10 and 6 million years ago. Surface uplift in the Central and Northern Andes since the late Miocene (roughly the last 10 million years) totals somewhere between 2,300 and 3,400 meters, at average rates of 0.2 to 0.3 millimeters per year.
This staggered timing means that while the Andes look like one continuous chain, their individual segments have distinct geological histories and reached their current elevations at very different moments.
How Scientists Determine the Timeline
Pinning dates on mountain-building events millions of years old requires several overlapping methods. One of the most important is uranium-lead dating of zircon crystals, tiny minerals that form in igneous and metamorphic rocks and lock in radioactive uranium at the moment of their creation. By measuring how much of that uranium has decayed into lead, scientists can calculate the crystal’s age with high precision. A specialized technique called SHRIMP (Sensitive High Resolution Ion Microprobe) allows researchers to date individual grains of zircon, which has been critical for establishing when sediments in the Patagonian Andes were originally deposited.
Paleobotany offers another window. Fossil plants found at high elevations can reveal when a surface was lifted above the altitude where certain species could survive. In the Colombian Andes, plant fossils helped establish the rapid uplift between 5 and 2 million years ago. Rare fossil finds, including Devonian-era trilobites and Late Triassic bivalves in southern Chile, help anchor the ages of the oldest basement rocks beneath the range. Other dating methods, including potassium-argon and rubidium-strontium techniques applied to metamorphic rocks and the granites that cut through them, fill in additional pieces of the timeline.
How the Andes Reshaped a Continent
As the Andes reached sufficient height, they fundamentally altered South America’s climate and river systems. The most significant shift came in the late Miocene, when the easternmost flanks of the range rose high enough to block moisture-laden air from the Pacific. This created a rain shadow effect, triggering aridification on the western side (contributing to the extreme dryness of the Atacama Desert) while funneling rainfall eastward into the Amazon basin. Fossil pollen records show a sharp increase in arid-adapted plant species during this period, coinciding directly with the uplift of the eastern Andean flanks.
The towering Central and Northern Andes also became the primary sediment source for the massive Pebas wetland system in the western Amazon, which existed before the modern Amazon River took its current form. The rise of the Andes essentially reorganized South America’s drainage, redirecting rivers that once flowed westward or northward into the transcontinental eastward flow of the Amazon system we know today.
The Andes Are Still Rising
The Nazca plate continues subducting beneath South America at roughly 5 to 8 centimeters per year, and the Andes remain one of the most seismically and volcanically active mountain ranges on Earth. The same forces that built the range over tens of millions of years are still at work. Whether the Andes are currently in an active growth spurt or a quiet period between pulses is harder to say, but the tectonic engine driving them upward has not shut off. The range’s frequent earthquakes and active volcanoes are direct evidence of ongoing mountain-building processes deep below the surface.